This invention relates to fuel controls, in particular, fuel controls for gas turbine engines.
The steady state operating characteristics of the typical gas turbine engine are such that as engine speed (N) increases the ratio between fuel flow (W.sub.f) and compressor discharge pressure (CDP) must be increased. The ratio, W.sub.f /CDP, is commonly referred to as ratio units. At low engine speeds, near idle, for example, another important characteristic of the gas turbine engine is that the differences between ratio units at different speeds are quite small. In other words, the steady state curve is substantially horizontal in the idle regions; in fact, it actually may have a reverse slope at low speeds.
The principal purpose for a gas turbine engine fuel control is to schedule fuel flow to the engine in a prescribed relationship to compressor discharge pressure in response to the power demands and changes made through a power lever. Generally speaking, the fuel control varies the ratio units in relation to engine power settings: During acceleration and high power demand conditions the ratio units are high; during low power operating conditions, such as deceleration, the ratio units are small. Thus, the fuel control may be viewed as scheduling the ratio units between prescribed minimum and maximum levels. For any particular power lever advance (PLA) the relationship between ratio units and engine speed ideally is constant. The intersection of the engine steady state line and the ratio unit line occurs at the engine's steady state speed for that PLA, which thus defines the particular ratio unit at that speed. At moderate and high engine speeds the intersection occurs a significant angle. Consequently, the differences between ratio units for different speeds is quite significant, and, as a result, minor variations in ratio units do not produce significant changes in engine speed. Thus, engine speed accuracy is quite high in these regions. However, at the lower engine speeds, where the steady state curve is flat or horizontal, the intersection angles are much smaller and the operating points therefore are not nearly as well defined. Thus, at lower speeds, the engine speed accuracy may be poor, making it difficult to obtain a desired speed at a particular PCA position.
The prior art focuses on numerous ways to alleviate these accuracy problems. In general, solutions have focused on scheduling minimum ratio units at the lower engine speeds through the use of a mechanical governor in the fuel control; by scheduling a minimum fuel flow for different low power (idle) settings, the intersection angles are increased dramatically. Above idle speeds, governor operation is not necessarily utilized, however, and the ratio units may be programmed in response to CDP alone. One example of a governor system is shown and described in my U.S. Pat. No. 3,611,719 for a FUEL CONTROL, which issued on Oct. 12, 1971 and is commonly owned herewith.
An additional constraint imposed upon gas turbine fuel controls is the need for an absolute minimum fuel flow to the engine for any PLA beyond a shutdown position. Typically, this has been achieved by providing supplemental fuel flow control circuits which act in conjunction with the minimum fuel flow provided by the mechanical governor, that simply establishes different minimum fuel flow in the idle region for different low power PLA settings.
Another function of a gas turbine fuel control is to modify fuel flow to the engine in relation to various parameters, such as engine speed, acceleration, and temperature, and ambient temperature and pressure. The reason for this is two-fold: to increase engine operating efficiency and to prevent engine operation at certain speeds and ratio units so as to avoid operation in the engine surge region. Recent advances in fuel controls are marked by increased use of electronic interfaces with hydromechanical fuel controls to provide these fuel flow modification characteristics. Principal among these recent advances is the use of computer based systems which sense the various parameters to provide signals which modify fuel control in the hydromechanical portion of the system. In as much as reliability is a principal factor in all fuel controls, it continues to be considered important to provide for engine operation separate and apart from the electronic portion. In other words, the electronic portion should not be used as the sole means for controlling the fuel control but, instead, as a means for modifying a basic control provided by the hydromechanical portion.
The foregoing techniques for achieving stable, efficient engine operation and establishing minimum fuel flow add considerably to the cost, maintenance and size of the fuel control system. Thus, while performance of such controls has been excellent, there is a distinct need for smaller, lighter fuel controls that accomplish the same results at significantly less cost. This is especially true for fuel controls for small turbine engines, such as those used on small, private jets and the like.
Frequently, another function, performed by the fuel control, especially more recent types, is preventing engine overboost operation. Generally this is done by limiting the fuel flow to the engine to a maximum level which is adjusted down in a prescribed relationship to a number of engine operating parameters, such as engine temperature, speed, compressor bleed. Contemporary approaches, however, are characteristically similar in that they limit fuel flow independently of actual power lever position. The most immediate consequence of this is a dead band region in the available power lever advance range. To the operator of the engine, this appears as a range of available power lever movement for more power, but no further engine power is actually available. Essentially, the dead band then is nothing more than a flat spot in the PLA-N curve. It can make engine control difficult because the operator may attempt to apply more power by advancing the lever from an intermediate position, yet no more power is available because the engine may actually be at its maximum power.
In dual engine installations the fuel controls supply fuel to their respective engines as a function of its engine's operating parameters. Contemporary engine control systems provide for engine synchronization by the controlling mechanical connection between the power lever and the fuel control. The speed of one engine is sensed as a reference speed and the power lever of the other engine is moved so as to adjust its speed to that of the reference. The accuracy and reliability of such systems is indirectly limited by such factors as mechanical hysteresis and control linkage dead band.